1. Field of the Invention
The present invention relates to a solid oxide fuel cell which generates electric power by the use of a flat plate-shaped solid oxide fuel cell element and a method for producing the solid oxide fuel cell.
2. Related Art Statement
Recently, fuel cells have been recognized as power generating equipment. The fuel cell is capable of directly converting chemical energy possessed by fuel to electric energy. Since the fuel cell is free from limitation of Carnot's cycle, the cell is a very promising technique owing to its high energy conversion efficiency, wide latitude of fuels to be used (naphtha, natural gas, methanol, coal reformed gas, heavy oil and the like), less public nuisance, and high electric power generating efficiency without being affected by scale of installation.
Particularly, since the solid electrolyte fuel cell (hereinafter referred to as "SOFC") operates at high temperatures of 1,000.degree. C. or so, activity of electrodes is very high. Thus, no catalyst of a noble metal such as expensive platinum is necessary. In addition, since the SOFC has low polarization and relatively high output voltage, its energy conversion efficiency is conspicuously much higher than those of the other fuel cells. Furthermore, since their constituent materials are all solid, the SOFC is stable and has long service life.
Since such fuel cells can be constructed by solid structural materials, various kinds of cell structures have been proposed. Among the so-called flat plate-shaped solid oxide fuel cells are structurally very promising because their electric power per unit volume can be easily increased.
With the flat plate-shaped SOFC and monolithic SOFC, however, there are problems in producing techniques and the like as follows.
An ion-conductive film of zirconia has been formed on a flat plate-shaped porous support by means of chemical vapor deposition (CVD), physical vapor deposition (PVD) or electrochemical vapor deposition (EVD). However, apparatuses for use in CVD, PVD and EVD are generally expensive and large in size. Moreover, film forming speeds are low so that these methods are low in productivity and not suitable for mass production and it is difficult to obtain films of wide areas. Furthermore, internal resistance in the cells is high owing to diffusion resistance of fuel gases passing through porous supports.
A method has been proposed to form corrugated green sheets of an ion-conductive film of zirconia, an air electrode film and a fuel electrode film, press contact these green sheets and sinter them. In this method, however, since green sheets of different materials are sintered together, it is difficult to adjust their shrinkages during firing. Further, when the green sheets are press contacted with one another, the green sheets are likely to be cracked or fractured. Moreover, insulating layers tend to be produced at interfaces between the ion-conductive film and the electrode films after they are sintered together.
In order to overcome these problems, Japanese Patent Application Laid-open No. 1-128,359 disclosed a method in which an ion-conductive plate of zirconia is formed by sintering, electrodes are formed on its both surfaces to form a flat plate-shaped SOFC element, and such SOFC elements and insulating spacers are alternately laminated to form a SOFC.
However, inventor's investigations revealed that these methods involved the following problems.
That is, in the SOFC using such so-called self-standing ion-conductive ceramic plates, the green sheet of the flat plate-shaped ion-conductive ceramic plate are formed by the tape-casting method. In operating such a laminated SOFC, the ion-conductive ceramic plate undergoes a difference in pressure between a fuel gas and an oxidizing gas in a fuel gas chamber and an oxidizing gas chamber provided on both sides of the ceramic plate, respectively In order to resist emergency gas purging, the ion-conductive ceramic plate must be made thick to prevent any cracks and fracture
On the other hand, in order to restrain an increase in resistance to the ion-conduction, the ion-conductive ceramic plate must be made thin. It is difficult, however, to fulfill the contradictory requirements. Therefore, the structural strength and internal resistance of the flat plate-shaped SOFC element are not sufficient up to now.